Physiological and environmental constraints to winter forage crops production

Abstract

A study was conducted on three forage species; faba bean, oats and Italian ryegrass, sown and harvested at different dates during the 2008 and 2009 growing seasons with the main objective being to maximize dry matter (DM) production. The general aim was to produce a total of 45 t DM ha⁻¹ yr⁻¹. The crops were sown on 4th March, 28th March, 21st April, 12th May and 3rd June in 2008 on a Templeton soil and on 16th March, 16th April and 15th May 2009 on a Wakanui silt-loam soil. Maximum total DM yield (TDM) of forage crops was higher in 2009 (from 15,995 to 23,055 kg ha⁻¹) compared with 2008 (from 6,360 to 13,490 kg ha⁻¹). This was essentially due to disease, bird damage, lodging, frost damage and snow fall in 2008 and a longer growing duration in 2009. Oats produced the highest maximum TDM yield in the 2008(12,010 kg ha⁻¹) and 2009 (23,055 kg ha⁻¹) growing seasons. This was followed by Italian ryegrass (8,720 kg ha⁻¹) and faba bean (7,580 kg ha⁻¹) in 2008 and faba bean-oat intercrop (21,155 kg ha⁻¹), faba bean (19,645 kg ha⁻¹) and Italian ryegrass (15,995 kg ha⁻¹) in 2009. In 2008, the weighted mean absolute growth rate (WMAGR) at earlier sowing dates was lower than at later sowing dates as was the maximum crop growth rate (Cmax). In contrast, the duration of crop growth (DUR) was longer for the earlier sowing dates than for the later sowings. This indicated that although the earlier sowing dates had lower WMAGR it occurred over a longer duration, thus the earlier sowing dates still produced a greater maximum TDM yield. However, the differences in maximum TDM in 2009 were influenced by the favourable growing conditions and there were no differences in the WMAGR and Cmax in this year. However, the linear growth phase of Italian ryegrass was longer than for the two other crops and an intercrop. Based on these results, oats was the best crop to sow for the lactation of dairy cows as it attained a herbage ME of 11.5 MJ kg⁻¹ DM and produced the greatest total ME per ha when sown in early March and harvested at the end of August. Oats were also suitable to sow for liveweight gain for sheep and cattle when harvested in mid October and they reached 10 MJ kg⁻¹ DM of ME. Italian ryegrass was the other alternative forage crop for farmers to sow. Faba bean was the best crop to choose for lactating dairy cows as it reached a required herbage N of 2.4% (15% CP). Similarly, Italian ryegrass (sown in early to mid March) was another option as it attained a N content of 2.4% (15% CP) when harvested in mid October. In summary, total DM yield can be maximized by sowing oats and Italian ryegrass in early or in the middle of March and sowing faba bean in the middle of March. The harvest dates of these forage crops should be in the end of August to the end of November depending on the choice of crop and desirable target. In relation to crop development, the findings of the present study indicate that using soil temperature taken at 20 mm depth was appropriate for calculating thermal time (Tt) requirements for emergence for all species. However, the use of air temperature for faba bean and oats and soil temperature for Italian ryegrass was most appropriate for calculating the Tt requirement at the leaf appearance stage. In addition, using air temperature was found to be appropriate at flowering/anthesis and pod filling stages. Based on the present study, the prediction of the following phenological phases; emergence, leaf appearance, flowering/anthesis and pod filling using Tt provides useful information to be extrapolated to other sites and seasons to determine the most suitable sowing and harvest dates as well as forage crop species to be sown. Total intercepted PAR in 2008 (398 to 589 MJ m⁻²) was lower than in 2009 (788 to 982 MJ m⁻²). This was mostly due to the shorter growth duration in 2008 which was associated with a shorter duration of LAI development and allowed a lower PAR to be intercepted. In addition, the lower incident solar radiation received, the occurrence of snow, disease and frost damage in 2008 compared with 2009 also influenced the amount of total intercepted PAR. Further, faba bean sown on the last three sowing dates did not reach canopy closure, nor did oats sown on the last two sowing dates. This meant that these crops captured less incident radiation than they could have. All of these causes could explain the variation in maximum TDM produced in both years. Higher RUE in the earlier sowing dates specifically for faba bean and Italian ryegrass was related to the higher temperature in the earlier sowings than in later sowings when the temperature was lower. However, for oats temperature did not affect RUE. In both study seasons, oats had a higher RUE (2.4-2.5 g DM MJ⁻¹) than faba bean (1.7-2.1 g DM MJ⁻¹), Italian ryegrass (1.8-2.1 g DM MJ⁻¹) and the faba bean-oat intercrop (2.3 g DM MJ⁻¹). To maximize total annual DM production the choice of a summer crop is also important. As shown in the present study, oats yielded 15 t DM ha⁻¹ when sown in early March (4th March) and harvested at the end of September. If this was followed by a summer crop of maize sown in October, it would give an approximate potential yield of 43 t DM ha⁻¹ yr⁻¹ which is close to the industry target of 45 t DM ha⁻¹ yr⁻¹. This was the best crop sequence found in the present study.